U.S. patent application number 11/584454 was filed with the patent office on 2008-04-24 for milling system and method of milling.
This patent application is currently assigned to Smith International, Inc.. Invention is credited to Harshad Patil, Malcolm Perschke.
Application Number | 20080093076 11/584454 |
Document ID | / |
Family ID | 38813993 |
Filed Date | 2008-04-24 |
United States Patent
Application |
20080093076 |
Kind Code |
A1 |
Patil; Harshad ; et
al. |
April 24, 2008 |
Milling system and method of milling
Abstract
A mill assembly includes a shaft; a lead mill secured to a first
end of the shaft, the lead mill including a first body having a
plurality of first blades and a plurality of first cutters having
substantially cylindrical bodies secured to the plurality of first
blades; and a second mill secured to the shaft a selected distance
from the lead mill, the second mill including a second body having
a plurality of second blades and a plurality of second cutters
having substantially cylindrical bodies secured to the plurality of
blades.
Inventors: |
Patil; Harshad; (Houston,
TX) ; Perschke; Malcolm; (Spring, TX) |
Correspondence
Address: |
OSHA, LIANG LLP / SMITH
1221 MCKINNEY STREET, SUITE 2800
HOUSTON
TX
77010
US
|
Assignee: |
Smith International, Inc.
Houston
TX
|
Family ID: |
38813993 |
Appl. No.: |
11/584454 |
Filed: |
October 20, 2006 |
Current U.S.
Class: |
166/298 ;
166/55.7 |
Current CPC
Class: |
E21B 7/064 20130101;
E21B 10/567 20130101; E21B 29/06 20130101 |
Class at
Publication: |
166/298 ;
166/55.7 |
International
Class: |
E21B 43/11 20060101
E21B043/11 |
Claims
1. A mill assembly, comprising: a shaft; a lead mill secured to a
first end of the shaft, the lead mill comprising: a first body
having a plurality of first blades; and a plurality of first
cutters having substantially cylindrical bodies secured to the
plurality of first blades; and a second mill secured to the shaft a
selected distance from the lead mill, the second mill comprising: a
second body having a plurality of second blades; and a plurality of
second cutters having substantially cylindrical bodies secured to
the plurality of blades.
2. The mill assembly of claim 1, wherein a gage diameter of the
second mill is substantially the same as a gage diameter of the
lead mill.
3. The mill assembly of claim 1, wherein the plurality of second
cutters are complimentary to the plurality of first cutters.
4. The mill assembly of claim 1, wherein at least a portion of at
least one of the plurality of first and/or second cutters have a
protective coating thereon.
5. The mill assembly of claim 1, wherein at least a portion of at
least one of the plurality of first and/or second blades have a
protective coating thereon.
6. The mill assembly of claim 1, wherein at least a portion of at
least one of the first and/or second body have a protective coating
thereon.
7. The mill assembly of claim 1, wherein at least a portion of at
least one of the plurality of first and/or second cutters, the
plurality of first and/or second blades, and the first and/or
second body have a protective coating thereon, and wherein the
protective coating comprises at least one of AlTiN, AlCr, SiTiN,
and SiN.
8. (canceled)
9. The mill assembly of claim 1, wherein at least a portion of at
least one of the plurality of first and/or second cutters, the
plurality of first and/or second blades, and the first and/or
second body have a protective coating thereon, and wherein the
coating has a thickness ranging from about 6 to 8 microns.
10. The mill assembly of claim 1, wherein at least a portion of at
least one of the plurality of first and/or second cutters, the
plurality of first and/or second blades, and the first and/or
second body have a protective coating thereon, and wherein the
coating has a hardness of at least 1000 HV.
11. The mill assembly of claim 1, wherein at least one of the
plurality of first cutters and second cutters has a workface angle
ranging from about -5 to -40 degrees.
12. (canceled)
13. The mill assembly of claim 11, wherein at least one of the
plurality of first cutters and second cutters has a workface angle
of about -15 to -18 degrees.
14. The mill assembly of claim 1, wherein the distance between a
gage of the lead mill and a gage of the second mill is selected
based upon the relationship: F=(3E*I* .delta..sub.max)/ L.sup.3
wherein F is load; E is modulus of elasticity of the shaft; I is
the moment of inertia; .delta..sub.max is the maximum deflection of
the shaft; and L is the length of the shaft from a gage of the lead
mill to the second mill.
15. A method of milling a window in a tubular in a wellbore,
comprising: engaging a lead mill of a mill assembly against an
interior surface of the tubular, the lead mill secured to an end of
a shaft and comprising: a first body having a plurality of first
blades; and a plurality of first cutters having substantially
cylindrical bodies secured to the plurality of first blades;
rotating the mill assembly; moving the mill assembly along a
surface of a whipstock assembly as the lead mill cuts the window in
the tubular, thereby deflecting the lead mill and shaft outwardly
through the window in the tubular; and engaging a second mill of
the mill assembly against the window in the tubular, the second
mill secured to the shaft a selected distance from the lead mill
and comprising: a second body having a plurality of second blades;
and a plurality of second cutters having substantially cylindrical
bodies secured to the plurality of blades.
16. The method of claim 15, wherein the lead mill engages the
tubular such that at least one of the plurality of first cutters
form a workface angle with the tubular ranging from about -5 to -40
degrees.
17. The method of claim 15, wherein the second mill engages the
tubular such that at least one of the plurality of second cutters
form a workface angle with the liner ranging from about -5 to -40
degrees.
18. The method of claim 15, wherein a gage diameter of the second
mill is substantially the same as a gage diameter of the lead
mill.
19. The method of claim 15, wherein at least a portion of at least
one of the plurality of first and/or second cutters, the plurality
of first and/or second blades, and the first and/or second body
comprise a protective coating.
20. A method of designing a mill assembly, comprising: determining
characteristics of a lead mill, the lead mill comprising: a first
body having a plurality of first blades; and a plurality of first
cutters having substantially cylindrical bodies secured to the
plurality of first blades; determining characteristics of a shaft
having a first and second end, wherein the first end is adapted to
receive the lead mill and the second end is adapted to be
threadably connected to a drill assembly; determining
characteristics of an engagement point; and selecting a location on
the shaft for the engagement point to be placed.
21. The method of claim 20, wherein the location is selected based
upon the relationship: F=(3E*I* .delta..sub.max)/ L.sup.3 wherein F
is load; E is modulus of elasticity of the shaft; I is the moment
of inertia; .delta..sub.max is the maximum deflection of the shaft;
and L is the length of the shaft from a gage of the lead mill to
the engagement point.
22. The method of claim 20, wherein the engagement point is
selected from the group of a second mill, a stabilizer, a motor,
and drill string.
23. A mill assembly, comprising: a shaft; a lead mill secured to a
first end of the shaft, the lead mill comprising: a body having a
plurality of blades; and a plurality of cutters having
substantially cylindrical bodies secured to the plurality of
blades; and a protective coating disposed on at least a portion of
at least one of the body, the plurality of blades, and the
plurality of cutters.
24. The mill assembly of claim 23, further comprising: at least one
second mill secured to the shaft a selected distance from the lead
mill.
25. The mill assembly of claim 23, wherein at least a portion of
the second mill comprises a protective coating thereon.
26. The mill assembly of claim 23, further comprising: at least one
of a motor and a stabilizer secured to the shaft a selected
distance from the lead mill.
27. The mill assembly of claim 23, wherein the protective coating
comprises at least one of AlTiN, AlCr, SiTiN, and SiN.
Description
BACKGROUND OF INVENTION
[0001] 1. Field of the Invention
[0002] Embodiments disclosed herein relate generally to a downhole
mill assembly. More particularly, the embodiments disclosed herein
relate to a method of milling and a method of designing a mill
assembly.
[0003] 2. Background Art
[0004] When an existing cased oil well becomes unproductive, the
well may be sidetracked in order to develop multiple production
zones or redirect exploration away from an unproductive zone.
Generally, sidetracking involves the creation of a window in the
well casing by milling the steel casing in an area either near the
bottom or within a serviceable portion of the well. The milling
operation is then followed by the directional drilling of rock
formation through the newly formed casing window. Sidetracking
enables the development of a new borehole directionally oriented
toward productive hydrocarbon sites without moving the rig,
platform superstructure, or other above ground hole boring
equipment, and also takes advantage of a common portion of the
existing casing and cementing in the original borehole.
[0005] Thus, sidetracking is often preferred because drilling,
casing, and cementing the borehole are avoided. As mentioned above,
this drilling procedure is generally accomplished by either milling
out an entire section of casing followed by drilling through the
side of the now exposed borehole, or by milling through the side of
the casing with a mill that is guided by a wedge or "whipstock"
component.
[0006] The casing window is generally created with a combination of
mills mounted on a shaft or mandrel at the bottom end of a drill
string and wedging between the casing and a whipstock, which is
generally set in the hole in combination with the first milling
run.
[0007] The peripheral surface of mills is generally covered with
abrasive or cutting inserts made of hard material such as sintered
tungsten carbide compounds brazed on a steel shaft. The hardness of
the whipstock is generally designed so that minimum wear will be
generated by the rotation of mills peripheral surface onto the
whipstock face while the assembly is pushed and rotated against the
casing wall under deflecting action of the whipstock. The milling
action generally results from unbalanced pressures between the
mill(s) and the whipstock on one hand and the mill(s) and the
casing wall on the other hand.
[0008] U.S. Pat. No. 4,266,621, which is herein incorporated by
reference, describes a milling tool for elongating a laterally
directed window in a well casing. The disclosed system requires
three trips into the well, beginning with the creation of an
initial window in the borehole casing, the extension of the initial
window within a particular cutting tool, and the elongation and
further extension of the window by employing an assembly with
multiple mills. While the window mill is aggressive in opening a
window in the casing, the number of trips, typically three, to
accomplish the task is expensive and time consuming.
[0009] By integrating a whipstock into the milling operation and
directionally orienting the milling operation to a more confined
area of well casing, the number of trips required to effectively
mill a window in a well casing has been decreased. A whipstock
having an acutely angled ramp is first anchored inside a well and
properly oriented to direct a drill string in the appropriate
direction. A second trip is required to actually begin the milling
operations. Newer methods integrate the whipstock with the milling
assembly to provide a combination whipstock and staged sidetrack
mill, allowing for casing windows to be milled in one trip. The
milling assembly is connected at its leading tool to the top
portion of the whipstock by a bolt, which upon application of
sufficient pressure, may be sheared off to free the milling
assembly. The cutting tool employed to mill through the metal
casing of the borehole has conventionally incorporated cutters that
include at least one material layer, such as preformed or crushed
tungsten carbide, designed to mill pipe casing. Several such
one-trip milling systems include those described in U.S. Pat. Nos.
5,771,972, 6,102,123, 6,648,068, which are herein incorporated by
reference in their entirety.
[0010] Conventional milling systems are, however, unable to mill
windows in chrome casings, casings which are steadily increasing in
number of wells due to the number of wells in severe drilling
environments, such as severely corrosive environments, deep wells,
cold environments, and sea bottoms, that are more commonly drilled
due to exhaustion of easily drillable wells. Severe environmental
conditions typically include the presence of corrosive fluids with
dissolved gases including oxygen (O.sub.2), hydrogen sulfide
(H.sub.2S), and carbon dioxide (CO.sub.2) gasses. Due to the
exposure to the severely corrosive environments, many downhole
components are exposed to a variety of corrosion mechanisms such as
uniform corrosion, pitting, corrosion fatigue, sulfide stress
cracking, hydrogen blistering, hydrogen embrittlement, stepwise
cracking, wormhole attack, galvanic ringworm corrosion, heat
affected corrosion, mesa attack, raindrop corrosion, and erosion
corrosion, necessitating the use of corrosion resistant alloys
(CRA), frequently duplex chrome, in the downhole components
including casings. Other typically corrosion and/or erosion
resistant CRA-type materials include: (1) stainless steel including
conventional austenitic, martensitic, precipitation hardened,
duplex, and ferritic stainless steels; (2) precipitation hardened
and solid solution nickel-based alloys and nickel copper alloys;
and (3) cobalt-based, titanium, and zirconium alloys.
[0011] In a duplex 25% chrome casing, for example, although desired
corrosion resistance can be obtained, the material proves to be
difficult in handling, specifically, in cutting and machining. The
material tends to be abrasive to cutting tools, as well as leading
to work hardening, smearing, galling, and welding. Furthermore, in
cutting chips in a chrome casing, high asperity-junction
temperatures are frequent.
[0012] The difficulties associated with milling through a chrome
casing leaves many mature wells neighbored by significant
quantities of oil otherwise unreachable without the cost of either
pulling the chrome casings and recompleting the existing well or
forming a new well. The ability to sidetrack a well would not only
allow for a multilateral well, but would also allow for
sidetracking of a stuck tubular.
[0013] As reported in "A New Casing-Exit System for Duplex 25%
Chrome Casing Enables Economically Viable Redevelopment of Mature
Fields," OTC 17594 (2005), a specially designed milling system,
requiring a new motor design to avoid over-torque on the drill
assembly, was the first reported milling system able to cut an exit
in a chrome well. However, further improvements in milling systems
would allow for increased longevity of mills in an operational
environment that frequently leads to failure of mills by abrasion
and/or galling.
SUMMARY OF INVENTION
[0014] In one aspect, embodiments disclosed herein relate to a mill
assembly that includes a shaft; a lead mill secured to a first end
of the shaft, the lead mill including a first body having a
plurality of first blades and a plurality of first cutters having
substantially cylindrical bodies secured to the plurality of first
blades; and a second mill secured to the shaft a selected distance
from the lead mill, the second mill including a second body having
a plurality of second blades and a plurality of second cutters
having substantially cylindrical bodies secured to the plurality of
blades.
[0015] In another aspect, embodiments disclosed herein relate to a
method of milling a window in a tubular in a wellbore that includes
engaging a lead mill of a mill assembly against an interior surface
of the tubular, the lead mill secured to an end of a shaft and
including a first body having a plurality of first blades and a
plurality of first cutters having substantially cylindrical bodies
secured to the plurality of first blades; rotating the mill
assembly; moving the mill assembly along a surface of a whipstock
assembly as the lead mill cuts the window in the tubular, thereby
deflecting the lead mill and shaft outwardly through the window in
the tubular; and engaging a second mill of the mill assembly
against the window in the tubular, the second mill secured to the
shaft a selected distance from the lead mill and including a second
body having a plurality of second blades and a plurality of second
cutters having substantially cylindrical bodies secured to the
plurality of blades.
[0016] In another aspect, embodiments disclosed herein relate to a
method of designing a mill assembly that includes determining
characteristics of a lead mill, the lead mill including a first
body having a plurality of first blades and a plurality of first
cutters having substantially cylindrical bodies secured to the
plurality of first blades; determining characteristics of a shaft
having a first and second end, wherein the first end is adapted to
receive the lead mill and the second end is adapted to be
threadably connected to a drill assembly; determining
characteristics of an engagement point; and selecting a location on
the shaft for the engagement point to be placed.
[0017] In yet another aspect, embodiments disclosed herein relate
to a mill assembly, that includes a shaft; a lead mill secured to a
first end of the shaft, the lead mill including a body having a
plurality of blades and a plurality of cutters having substantially
cylindrical bodies secured to the plurality of blades; and a
protective coating disposed on at least a portion of at least one
of the body, the plurality of blades, and the plurality of
cutters.
[0018] Other aspects and advantages of the invention will be
apparent from the following description and the appended
claims.
BRIEF DESCRIPTION OF DRAWINGS
[0019] FIG. 1 shows a mill assembly according to one embodiment
disclosed herein.
[0020] FIGS. 2A and 2B show an enlarged side and top view of the
lead mill shown in FIG. 1.
[0021] FIG. 3 shows an enlarged side view of the second mill shown
in FIG. 1.
[0022] FIG. 4 shows a mill assembly according to one embodiment
disclosed herein
[0023] FIG. 5 shows a side view of a cutter to illustrate workface
angle.
[0024] FIG. 6 shows a side view of a cutter to illustrate rake
angle.
[0025] FIGS. 7A and 7B shows a cutter according to one embodiment
disclosed herein.
[0026] FIGS. 8 and 8A show a cutter according to one embodiment
disclosed herein.
[0027] FIGS. 9A and 9B show a milling and whipstock system which
may incorporate an embodiment of a mill assembly disclosed
herein.
DETAILED DESCRIPTION
[0028] In one aspect, embodiments disclosed herein relate to mill
assemblies that include a lead mill, a second mill, and a shank
therebetween on which each of lead mill and second mill are
attached. Embodiments disclosed herein may also relate to mill
assemblies having a single mill attached to the end of a shank, and
mill assemblies having a lead mill, one or more second mills, and a
shank therebetween. Further, embodiments disclosed herein may also
relate to methods of designing a mill assembly, and methods of
milling a window in a tubular. As used herein, "second mill" refers
to any type of mill, e.g., dress mill, watermelon mill, string
mill, follow mill, etc., that may elongate and/or dress the window
to full gage.
[0029] Referring now to FIG. 1, a mill assembly generally
designated as 100 is shown. Mill assembly 100 includes a lead mill
110, which is attached to the bottom end of shaft 120. Located
above and spaced a distance X from the lead mill 110 is a second
mill 130 that is also mounted on shaft 120. Shaft 120, as shown in
FIG. 1, includes a lower section between the lead mill 110 and
second mill 130 and an upper section, above the second mill 130.
The upper end of shaft 120 may be either threadably connected to a
drill string (not shown) or threaded to another subassembly (not
shown). As shown in FIG. 1, lead mill 110 has a gage 111 and second
mill 130 has a gage 131 such that the gage 111 of lead mill 110 is
substantially the same as the gage 131 of second mill 130. One of
ordinary skill in the art would recognize that in alternative
embodiments, the gage of the second mill 130 may be either larger
or smaller than the gage of lead mill 110.
[0030] Referring to FIGS. 2A and 2B, the lead mill 110 illustrated
in FIG. 1 is shown. Lead mill 110 includes a body 112 and a
plurality of blades 114 extending radially from the body 112. The
body 112 may be formed from steel or a tungsten carbide matrix
infiltrated with a binder alloy or any other material used in the
art. A plurality of cylindrically bodied cutters 116 are attached
to each of the plurality of blades 114 in cutter pockets 115,
typically by brazing. The blades 114 and the cutters 116 generally
form a cutting structure of the lead mill 110. Within the mill body
112 are one or more passages ending in openings 118 through which
drilling fluid may be delivered to cool the tool surface and remove
accumulated debris. Lead mill 110 may also include a threaded
connection (not shown) for attachment to the shaft 120 shown in
FIG. 1A. After threadedly engaging the lead mill 110 to the shaft
120, the connection may also be welded as known in the art.
[0031] Referring to FIG. 3, the second mill 130 illustrated in FIG.
1 is shown. Second mill 130 includes a body 132 and a plurality of
blades 134 extending radially from the body 132. The body 132 may
be formed from steel or a tungsten carbide matrix infiltrated with
a binder alloy or any other material used in the art. A plurality
of cylindrically bodied cutters 136 are attached to each of the
plurality of blades 134 in cutter pockets 135, typically by
brazing. The blades 134 and the cutters 136 generally form a
cutting structure of the second mill 130. In the embodiment shown
in FIGS. 1-2, second mill 130 is mounted on the shaft 120 via
connections (e.g. threaded connections) (not shown). Alternatively,
the lead mill 110 and/or second mill 130, including the body 112,
132 and blades 114, 134, may be integral with the shaft 120. In one
embodiment, lead mill and/or second mill may be formed from a solid
body having integral flow paths formed, for example, by machining,
formed therein. In another embodiment, lead mill and/or second mill
may be formed from a mold via an infiltration or casting
process.
[0032] Referring to FIG. 4, a mill assembly generally designated as
200 is shown. Mill assembly 200 includes a lead mill 210, which is
attached to the bottom end of shaft 220. Located above and spaced a
distance X from the lead mill 210 is a second mill 230 that is also
mounted on shaft 220. Shaft 220, as shown in FIG. 1, includes a
lower section between the lead mill 210 and second mill 230 and an
upper section, above the second mill 230. The upper end of shaft
220 may be either threadably connected to a drill string (not
shown) or threaded to another subassembly (not shown). As shown in
FIG. 4, second mill 230 is integral with shaft 220. In a particular
embodiment, lead mill and/or second mill may be formed from a solid
body having integral flow paths formed, for example, by machining,
formed therein.
[0033] As shown in FIGS. 1-3, the blades 114, 134 are spiral blades
positioned about the perimeter of the bodies 112, 132 at
substantially equal angular intervals. However, other blade
arrangements may be used with embodiments of the present disclosed,
and the embodiment shown in FIGS. 1-3 is not intended to limit the
scope of the embodiments disclosed herein. For example, the blades
114, 134 may be positioned at unequal angular intervals or be
straight instead of spiral.
[0034] As also shown in FIGS. 1-3, the mill assembly includes a
lead mill and a second mill. However, one of ordinary skill in the
art would recognize that in some embodiments, a mill assembly may
include multiple second mills, i.e., a third mill, etc., or in
other embodiments a mill assembly may instead include a lead mill
followed by a motor or stabilizer attached to the shaft a selected
distance above the lead mill, rather than a second mill.
Additionally, while lead mill 110 and second mill 130 are shown as
having cutters 116 and 136 disposed thereon, in some embodiments,
either lead mill 110 and/or second mill 130 may not contain cutters
attached thereto. In an alternate embodiment, either lead mill 110
and/or second mill 130 may include, instead of cutters, crushed
carbide welded thereon.
[0035] The cutters 116, 136 attached to lead mill 110 and second
mill 130, according to some embodiments disclosed herein, may
include tungsten carbide particles and a metal binder. Typical
types of tungsten carbide include cemented tungsten carbide
(crushed and spherical), cast tungsten carbide (crushed and
spherical), macrocrystalline tungsten carbide, and carburized
tungsten carbide. In a particular embodiment, the cutters 116, 136
may include crushed cemented tungsten carbide. For various
embodiments using cemented tungsten carbide, the cemented tungsten
carbide formed from carbide particles ranging in size from about
less than 1 to 15 microns and cobalt in amount ranging from about 6
to 16 percent by weight. However, such sizes and amounts are not
intended to be a limitation on the scope of the present invention.
One skilled in the art would recognize that in using a cemented
tungsten carbide, by varying the carbide particle size and/or
cobalt content, the wear resistance/hardness and fracture toughness
of the cutters may be optimized for a particular milling
operation.
[0036] In a particular embodiment, the cutters 116, 136 may be
formed from crushed cemented tungsten carbide particles ranging in
size from about less than 1 to 10 microns and cobalt in amount
ranging from 8 to 14 percent by weight. In various other
embodiments, the cutters may also include other particles such as,
for example, tantalum carbide, tantalum niobium carbide, titanium
carbide, tungsten titanium carbide, and tungsten tantalum
carbide.
[0037] In one embodiment, the cylindrically bodied cutters may be
attached to each of the lead mill and the second mill in such a
manner so as to provide for a desired workface angle between the
cutting face of the cutters and the workface (material being cut)
as the cutters engage the casing. The workface angle, shown as 13
in FIG. 5, may be defined as the angle subtended between a plane
520 of the cutting face 515 of the cutter 510 and a line 525
perpendicular to the contact point at the workface surface 530. In
a particular embodiment, at least some of the cutters on the lead
mill have a workface angle ranging from about -5 to -40 degrees,
from about -10 to -35 degrees in another embodiment, from about -15
to -18 in another embodiment, and a workface angle of about -15
degrees in yet another embodiment.
[0038] The desired workface angle may be achieved, for example, by
varying the cutter geometry or the placement of the cutters in
cutter pockets in the blades of a mill to achieve a particular rake
angle as known in the art. In one embodiment, the desired workface
angle may be achieved by placing cylindrical cutters in angled
cutter pockets. In another embodiment, the desired workface angle
may be achieved by forming cylindrical bodied cutters having a
cutting face angled with respect to the longitudinal axis of the
cylindrical body of the cutter. One of ordinary skill in the art
would recognize that one or more techniques may be used to achieve
the desired workface angle.
[0039] In one embodiment, the workface angle may be achieved by
varying the rake angle of the cutters. Rake angle, shown as .beta.
in FIG. 6, may be defined as the angle subtended between a plane
620 of the cutting face 615 of the cutter 610 and a line 625
parallel to the longitudinal axis of the mill (not shown). In one
embodiment, the cutters may be placed at an angle ranging from -5
to -50 degrees to achieve the desired workface angle, from -10 to
-35 degrees in another embodiment, and from -30 to -33 degrees in
yet another embodiment.
[0040] Referring now to FIGS. 7A-B, one technique for varying
cutter geometry to have the desired rake angle is to form an
axisymmetric cutter 700 having a cylindrical base 702. By cutting
cutter 700 on a plane 704 that forms an angle .theta. with respect
to a plane perpendicular to the axis of the insert 700, a top
portion 706 is generated, as shown in FIG. 7A-B. When top portion
706 is rotated 180.degree. and re-attached to base 702, it will be
canted with respect to base 702 at an angle that is equal to
2.theta.. Alternatively, cutter 700 having a top portion 706 canted
with respect to base 702 may be created by building up top portion
706 of cutter 700 from base 702.
[0041] Referring now to FIG. 8, another technique for varying
cutter geometry to have the desired rake angle is shown. Cutter 800
has a cylindrical base 802. By grinding or cutting away portion 808
of cutter 800 to form plane 804 having angle .theta. with respect
to a plane perpendicular to the axis of the insert 800, a top
portion 806 is generated, as shown in FIG. 8. Cutter 800 may be
ground either prior to or post insertion into the cutter pocket
(not shown) of the mill. In some embodiments, top portion 806 may
optionally include a tapered tip 812 on the side of the cutter 800
to be inserted into the cutter pocket (not shown).
[0042] In a particular embodiment, second mill cutters are
complimentary to lead mill cutters, i.e., lead mill cutters and
second mill cutters have substantially similar orientations with
respect to the workface.
[0043] In some embodiments, a protective coating may be provided on
a portion or all of various mill components of each of the lead
mill and second mill, including for example, the cutters, blades,
and bodies. In some particular embodiments, the coating may be
applied on the cutters prior to insertion and brazing into the
cutter pockets. In other embodiments, the coating may be applied to
the cutters after the insertion and brazing of the cutters into the
cutter pockets. In yet other embodiments, the coating may be
applied to all or any portion of the cutters, blades and mill body
before or after brazing the cutters. In a specific embodiment, both
the lead and second mill may individually have select portions
coated. Specifically, the cutters of the second mill may be coated
while the entire head of the lead mill may be coated.
[0044] In a particular embodiment, a protective coating may provide
an increase in hardness and surface lubricity (low coefficient of
friction) to the tool surface. Increases in hardness and/or surface
lubricity may provide for a reduction in abrasive wear, work
hardening, smearing, galling, and/or welding. Examples of coatings
suitable for use on the mill components as disclosed herein include
an aluminum titanium nitride (AlTiN) coating and an aluminum chrome
(AlCr) coating.
[0045] Other types of coatings that may provide increases in
hardness and/or lubricity that may be used on a mill component as
described herein may include titanium nitride, titanium aluminum
nitride, titanium carbonitride, aluminum oxide, chromium nitride,
aluminum chromium nitride, and chromium carbide. Other embodiments
may include silicon based coatings, such as, for example, silicon
nitride or silicon titanium nitride. In high heat applications, a
coating with a high oxidation temperature or high heat hardness or
a coating having aluminum therein may be desirable. Upon oxidation
of aluminum, aluminum oxide may serve as a heat barrier. In a
particular embodiment, the coating may include metal providing
strength to the coating and a lubricious material, such as
aluminum, fluoropolymers, including TEFLON.RTM. (E.I. DuPont de
Nemours Corporation, Wilmington Del.). One of skill in the art
would recognize that the selection of protective coating may be
dependent upon several factors including, for example, wear
resistance, surface lubricity, and oxidation temperature. In one
particular embodiment, the protective coating has a static
coefficient of friction of about 0.4 or less (against steel).
[0046] The coating may be applied by various techniques known in
the art, including physical vapor deposition (PVD), chemical vapor
deposition (CVD), plasma assisted chemical vapor deposition
(PACVD), and plating. One of ordinary skill in the art would
recognize that each coat of material may include multiple layers.
In applying the coatings, depending on the desired thickness of the
coating, the coating may include a single coat (or pass) of
material or multiple coats with one or more coating compositions.
In one embodiment, the coating may be applied to the lead mill
and/or second mill in a total thickness ranging from about 2 to 15
microns. In a particular embodiment, the coating may include two
coats of material, each coat ranging from about 3 to 4 microns, to
provide a total coating thickness ranging from about 6 to 8
microns.
[0047] In one embodiment, the protective coating may have a
hardness of at least about 1,000 HV. In another embodiment, the
protective coating may have a hardness of at least about 2,000
HV.
[0048] As shown in FIG. 1, second mill 130 may be located a
selected distance X on the shaft 120 above the lead mill 110.
Distance X, as shown in FIG. 1 is measured from the gage 111 of
lead mill 110 to the gage 131 of second mill 130. The distance X
may be selected to provide sufficient flexibility in shaft 120
between lead mill 110 and second mill 130 during a milling process.
The flexibility, which allows for the deflection observed in shaft
120, is dependent upon the characteristics of the shaft 120 and the
applied load according to the following relationship:
.delta..sub.max=(F*L.sup.3)/(3E*I) (1)
where .delta..sub.max is the maximum deflection; E is the modulus
of elasticity; I is the moment of inertia; F is the total load; and
L is the length of the shaft (i.e., the distance X in the above
discussion).
[0049] For a given set of characteristics of a mill assembly (ID,
OD of shaft, material type), the length of the shaft from the lead
mill to a tangential engagement or contact point with the casing
wall must be sufficient to allow for enough flexibility or
deflection in the shaft to mill a window in the casing under the
applied load without failure of the milling system. As used herein,
the "engagement point" refers to the point on the mill assembly or
BHA (or BHA component), i.e., second mill, motor, stabilizer, or
drill string, which touches the casing wall, as the mill assembly
begins to deflect and mill through the casing. For unit deflection
(.delta..sub.max=1) of a given shaft, the allowed load for a
particular mill assembly may be determined by:
F=(3E*I)/L.sup.3 (2).
[0050] Thus, the design of a mill assembly, and in particular the
distance between a lead mill and second mill or other type of
engagement point, may be selected or optimized in accordance with
the above relationship to allow for sufficient flexibility under
applied loads. In other words, in order to select a location for
the second mill, the above relationships may be used based on known
values (of inertia and modulus of elasticity). Alternatively, the
distance may be selected and a recommended load can be provided.
Those having ordinary skill in the art will appreciate that other
design considerations may also affect the ultimate placement of the
second mill or length of the shaft from the lead mill to the
engagement point.
[0051] In a particular example, in a 5 inch casing system (duplex
25% chrome casing) using a 2.25 inch OD, a 1.25 inch ID, and a 58
inch long shaft, unit deflection of the shaft may be achieved under
an applied load of at least 525 pounds. One of ordinary skill in
the art would recognize that for a given mill system, that as the
length of the shaft is increased, the applied load that will result
in unit deflection may decreased. In another embodiment, in a 5
inch casing system using a 2.25 inch OD, a 1.25 inch ID, and a 180
inch long shaft, unit deflection of the shaft may be achieved under
an applied load of at least 17.56 pounds. Further, one of ordinary
skill in the art would also recognize that for a given force
required for unit deflection, the moment of inertia (f[OD,ID]) and
length of the shaft may be varied in accordance with Equation 2
above to result in the same force for unit deflection.
Additionally, one of ordinary skill in the art would recognize that
there may be a critical load needed for unit deflection after which
the mill may fail to cut open a window in the casing and instead
cut into the whipstock.
[0052] In a particular embodiment, a mill assembly such as the one
disclosed herein may be included a one-trip milling/whipstock
system, such as those described in U.S. Pat. Nos. 5,771,972,
6,102,123, 6,648,068, which are herein incorporated by reference in
their entirety. Briefly, a one trip mill system, as shown in FIGS.
8A and 8B include a milling assembly generally designated as 30 and
a whipstock assembly generally designated as 60 that includes a
whipstock 44. The mill assembly 30 includes a lead mill generally
designated as 32, which is attached to the bottom end of a shank or
shaft 31. Located above and spaced from the lead mill 32 is, for
example, a second or follow mill 33 that is also mounted to the
shaft 31. The upper end of the shaft 31 is either threadably
connected to a drill string or threaded to another subassembly (not
shown). A tubular member 27 may form the shaft 31 on which mills 32
and 33 are mounted. Tubular member 27 may include a lower reduced
diameter portion on which mill 32 is disposed with mill 33 being
disposed on the fill diameter of tubular member 27. This reduction
in diameter provides flexibility between mills 32 and 33 during the
milling process.
[0053] Blade 38 immediately adjacent the parallel surface 45 of
whipstock 44 may be sufficiently wide to accommodate the shear bolt
39 threaded into the blade 38. The head of the shear bolt 39 is
seated in the top of the whipstock 44 and the shank 54 of the shear
bolt 39 is threaded into blade 38. The shank 54 may be hollow so
that, once the bolt 39 is sheared, the shank 54 serves as a nozzle
extension for nozzles 69 positioned at the base of shank 54 and at
the entrance to flex conduit 37 that directs fluid to the whipstock
anchor (not shown).
[0054] The whipstock 44 has a diameter D.sub.W that approximates
the inside diameter D.sub.I of the interior wall of casing 11 which
allows whipstock 44 to be lowered through cased borehole 9.
Whipstock 44 also includes a profiled ramp surface 28 having a
curved or arcuate cross section and multiple surfaces, each of the
multiple surfaces forming its own angle with the axis 26 of
whipstock 44. Profiled ramp surface 28 includes a starter surface
45 having a steep angle preferably 15.degree., a vertical surface
46 preferably parallel to the axis 26, an initial ramp surface 47
having a standard angle ranging from about 0.5 to 3.degree., a
"kick out" surface 48 having a steep angle preferably 15.degree.,
and a subsequent ramp surface 49 having a standard angle ranging
from about 0.5 to 3.degree.. It should be appreciated that these
angles may vary. For example, the starter ramp surface 45 may have
an angle A in the range of 1 to 45.degree. in one embodiment, 2 to
30.degree. in another embodiment, 3 to 15.degree. in yet another
embodiment, and about 15.degree. in still another embodiment. The
vertical surface 46 may have a length approximately equal to or
greater than the distance between mills 32 and 33. In a particular
embodiment, ramp surfaces 46, 49 may range from greater than zero
to 15.degree.. One of ordinary skill in the art would recognize
that the surfaces angles may be selected depending on the desired
window dimensions.
[0055] The backside 62 of the whipstock 44, especially adjacent the
upper end of the whipstock 44, is contoured to conform to the
inside diameter D.sub.I of the interior wall of the pipe casing 11
for stability of the top of the whipstock 44. The opposite lower
end of the whipstock 44 is secured to, for example, a hydraulically
actuated anchor (not shown). A typical anchor is shown in U.S. Pat.
No. 5,657,820, incorporated herein by reference in its
entirety.
[0056] The mill 32 and whipstock 44 disclosed herein are configured
such that the mill 32 tends to cut the wall of the casing 11 and
not the whipstock 44. To achieve this objective, various factors
are taken into consideration including the contact area and contact
stress between the mill 32, casing 11, and whipstock 44 and the
cutability of the metal of the casing and of the metal used for the
whipstock 44.
[0057] Advantageously, embodiments disclosed herein may provide for
at least one of the following. Mill assemblies incorporating
cylindrical cutters on each of the lead and second mill may allow
increased mill efficiency and mill life for ease in modification of
existing mill assemblies. By forming a mill assembly that has a
second mill distance from the lead mill that is selected to allow
for flexibility, a mill assembly as disclosed herein may be
effective in cutting a window in a casing that would otherwise be
unobtainable, without otherwise altering the mill and drill
assembly components.
[0058] When milling through a casing, one potential mode of failure
of the mill is by galling and welding observed at the mill face,
especially when milling through a chrome casing. The inclusion of a
protective coating on the cutting structure may prevent or reduce
such occurrences of galling. In the various embodiments that may
include a lubricious material such as aluminum or silicon in the
protective coating, the coating may also have an increased life
span due to the preferential oxidation of aluminum, which further
reduces galling and welding and contact temperatures.
[0059] Additionally, by coating the cutting structure, a dramatic
increase in wear resistance of the mill may be observed. The mill
assemblies disclosed herein may endure significant increases in
downhole life as compared to a typical mill assembly, and even when
tripped, mill assemblies made in accordance with embodiments
disclosed herein, and specifically the gage of the mills disclosed
herein, may have worn minimally while downhole.
[0060] Furthermore, as the cutting structure meets the casing wall,
the cutting structure typically encounters severe vibrations that
frequently lead to cracks in the cutters. By varying the cutter
geometry and/or placement of cutters with respect to the workface
surface, i.e., interior casing surface, the incidence of cracking
may be decreased and the cutting efficiency, and thus mill life,
may be increased. Additionally, by varying the cutter geometry by
grinding or cutting the cutters to have the desired rake and
workface angles, the number of cutters on a particular mill may be
increased, and thus the amount of wear each cutter experiences may
decrease.
[0061] While the invention has been described with respect to a
limited number of embodiments, those skilled in the art, having
benefit of this disclosure, will appreciate that other embodiments
can be devised which do not depart from the scope of the invention
as disclosed herein. Accordingly, the scope of the invention should
be limited only by the attached claims.
* * * * *